wifi: mt76: mt7996: revise TXS size

Size of MPDU/PPDU TXS is 12 DWs.
In mt7996/mt7992, last 4 DWs are reserved, so TXS size was mistakenly
considered to be 8 DWs. However, in mt7990, 9th DW of TXS starts to be used.

Signed-off-by: Benjamin Lin <benjamin-jw.lin@mediatek.com>
Link: https://patch.msgid.link/20250311103646.43346-1-nbd@nbd.name


Signed-off-by: Felix Fietkau <nbd@nbd.name>